Worcester Greenstar CDi Sealed System Components: Complete Part Number Reference

The Worcester Greenstar CDi range has been a workhorse in British homes for over a decade. We've seen thousands of these boilers come through our service schedules, and one thing remains constant: when you need a part, you need the right part. No guesswork, no returns, and no frustrated callbacks from homeowners whose heating is still down.

This reference guide cuts through the confusion. We've catalogued the core sealed system components across the CDi range, from the 24kW compact units right up to the 42kW models, with exact part numbers and compatibility notes. It's the kind of real-world context you won't find in a standard parts catalogue.

Understanding the CDi Sealed System Architecture

Worcester designed the Greenstar CDi series around a sealed system configuration, which means the system operates under pressure without needing a feed and expansion tank in the loft. The boiler itself contains several critical components that maintain this environment while managing combustion and water circulation. Heating and Plumbing World stocks these essential components to keep your installations running efficiently.

The beauty of the CDi design lies in its modular construction. Most components can be replaced individually without dismantling half the boiler. However, Worcester made running changes throughout the CDi production run. A 2008 CDi 30 doesn't use identical parts to a 2012 model, even though they share the same output rating. Always cross-reference against the boiler's data badge before ordering.

Heat Exchanger Components

The primary heat exchanger represents the heart of any condensing boiler. In the CDi range, Worcester uses aluminium silicon alloy heat exchangers with a finned design that maximises surface area for heat transfer.

• Part Number 87161463450: This is the primary heat exchanger for CDi 24, 28, and 30 models manufactured between 2005 and 2012. It includes the burner gasket and securing clips.
• Part Number 87161463460: The 35CDi and 42CDi models use this larger exchanger to handle increased heat loads.

Never attempt to clean a CDi heat exchanger with aggressive descalers. The aluminium alloy corrodes rapidly under acidic conditions. If it's scaled beyond a gentle flush, replacement is the only reliable option.

Gas Valve Assembly

The gas valve controls fuel delivery to the burner, modulating output to match demand. Worcester specified Honeywell valves across the CDi range. These are known for reliability but can occasionally stick after prolonged inactivity.

Part Number 87161165680 covers the CDi 24, 28, and 30 models. This is a complete assembly including the valve body and stepper motor. For the 35CDi and 42CDi, you'll need Part Number 87161165690. The larger models require a valve with higher flow capacity. If you fit the wrong one, you'll either underfire the boiler or cause potential exchanger damage.

Pump and Hydraulic Components

The circulation pump moves heated water through the system. Worcester fitted Grundfos pumps as original equipment, and replacement pumps maintain that specification. Part Number 87161409150 is the standard pump for CDi 24-35 models.

On a service call last winter, I found a CDi 30 where the pump had seized solid after just four years. The homeowner had ignored the "black water" in the system and hadn't added inhibitor after a radiator move. The grit had acted like liquid sandpaper on the internal bearings, which turned a simple maintenance check into an expensive component replacement. It's always worth checking the Worcester Bosch condensate pump at the same time to ensure the entire hydraulic circuit is clear.

The diverter valve directs flow between heating and hot water. Part Number 87161431140 fits all CDi sealed system models. It's a motorised three-port valve with a reputation for longevity.

Pressure and Safety Components

The expansion vessel absorbs pressure fluctuations as water heats and cools. Worcester fits an internal vessel in the CDi range, which is accessible from the front panel.

Part Number 87161190060 is the standard 8-litre Worcester boiler expansion vessel for CDi 24-30 models. Larger models use a 10-litre vessel, Part Number 87161190070. A failed expansion vessel usually shows up as frequent pressure loss.

The pressure relief valve (PRV) protects against over-pressure. Part Number 87161190080 is universal across the range and rated at 3 bar. We also recommend replacing the Worcester condensing trap washer set if you notice any moisture around the safety discharge area.

Control Board and Electrical Components

The printed circuit board (PCB) is the boiler's brain. It manages the ignition sequence, modulation, and safety lockouts. Think of the PCB like a conductor of an orchestra; it ensures every component, from the fan to the gas valve, plays its part at exactly the right moment to create a harmonious output.

Part Number 87483002350 is the control board for CDi models manufactured before mid-2010. Later models use Part Number 87483002360, which adds OpenTherm compatibility. These boards aren't interchangeable, so match the replacement to the original specification. If you're upgrading the user interface, you might consider a wireless combi pack for better control.

Combustion and Flue Components

The fan assembly creates the pressure differential needed for combustion. Part Number 87161166250 covers CDi 24-30 models. Fan failures typically occur after 10-15 years and are often preceded by bearing noise.

The air pressure switch monitors fan operation and prevents ignition if airflow is insufficient. Part Number 87161166270 is universal across the range. If the boiler won't attempt ignition, this is often the culprit. You should also check the Baxi temperature sensor or equivalent NTC probes if the boiler locks out on overheat.

Conclusion

The Worcester Greenstar CDi sealed system range remains highly serviceable despite many models being fifteen years old. Three principles guide reliable maintenance: use genuine Worcester parts, verify compatibility against the serial number, and maintain the system with annual inhibitor checks. If you need help identifying a specific part, contact our technical team for expert assistance.